Solar cells that generate power by converting light energy into electric energy nave long been used. The main source of the light energy is sunlight.
For example, a solar cell is formed by stacking a p-type semiconductor layer and an n-type semiconductor layer one on top of the other. When the solar cell is exposed to light such as sunlight, electron-hole pairs are generated at the on junction, and an electromotive force is thus generated.
Generally, solar cells are formed, using inorganic-based or organic-based materials. For inorganic-based solar cells, mostly Si is used as the inorganic semiconductor material. The polarity type, p or n, is controlled by adding impurities to Si.
For organic-based solar cells, organic compounds such as organic dyes, organic polymers, etc. or carbon nanotubes are used as the organic semiconductor material. Solar cells are formed by depositing such materials on a substrate made of an electrically insulating material such as glass.
[Patent Document 1] Japanese Unexamined Patent Publication No. S58-78066
[Patent Document 2] Japanese Unexamined Patent Publication No. H09-15353
[Patent Document 3] Japanese Unexamined Patent Publication No. 2007-81097